1. Field of the Invention
The invention relates to the use of silicon oxide compounds as free-flow agents in the production of solid polyvinyl acetate resins.
2. Description of the Related Art
In this application, solid resins are understood as meaning vinyl acetate polymers which are present in solid form and are produced by means of mass polymerization or suspension polymerization. The solid polyvinyl acetate resins obtainable by means of mass polymerization are used in particular in the production of chewing gum material, while the solid polyvinyl acetate resins produced by means of suspension polymerization are used for the production of moldings, adhesives and finishes and as low-profile additives.
A disadvantage is that the solid polyvinyl acetate resins generally present in the form of pellets or as powder tend to agglomerate owing to their relatively low glass transition temperature Tg or their surface tack and buildup of electrical charge, which complicates the use of solid polyvinyl acetate resins as raw materials for further processing. The storage limit of solid polyvinyl acetate resins is therefore reached at a temperature as low as about 25° C.
It is known from the area of polymer powders redispersible in water that their flow behavior can be improved by addition of anticaking agents. Redispersible polymer powders, for example based on polyvinyl acetate, are obtained by polymerization in an aqueous medium, and the polymer dispersion obtained thereby is dried after addition of water-soluble protective colloid, usually polyvinyl alcohol. The polymer particles are therefore present with a relatively “hard” protective colloid covering, which firstly ensures the redispersibility in water. It also prevents caking, so that such powders can remain stable during storage up to temperatures of 60° C. DE 197 32 333 A1 recommends a mixture of hydrophilic and hydrophobic silicic acid as an anticaking agent for redispersible polymer powders. DE 195 45 608 A1 and DE 103 17 882 A1 recommend a multiplicity of anticaking agents for redispersible polymer powders, such as carbonates, talc, gypsum, silicic acid, kaolins, hydrophobically modified silicic acid and hydrophobically modified silicon oxide.
DE 102 51 790 A1 shows that relatively hydrophilic solid resins, such as polyamides, show no agglomeration with silicic acid which has been rendered hydrophobic, whereas lumpy powder is obtained with hydrophilic silicic acid.